1. Field of the Invention
The present invention relates to a signal-processing method, which specifically relates to a signal-processing method for filtering a data obtained by measuring surface texture of a workpiece such as dimension, shape, waviness and roughness.
2. Description of the Related Art
Measuring instruments for measuring surface texture of a workpiece e.g. profile, roughness and waviness, such as a coordinate measuring machine for measuring three-dimensional shape of a workpiece, a form measuring machine and an image measuring machine for measuring two-dimensional profile, a roundness measuring machine for measuring roundness and a surface roughness tester for measuring waviness and roughness of a workpiece surface have been conventionally known. In such measuring instruments, a contact-type or non-contact type sensor and a workpiece are relatively moved to collect measurement data of the workpiece surface.
Usually, disturbance such as noise is contained in thus collected measurement data.
The disturbance is usually an electric and magnetic induction noise containing high-frequency components, however, when the profile of the workpiece surface is to be measured, surface roughness and waviness can be the disturbance.
In order to eliminate the disturbance as necessary, the measurement data is often filtered to remove, for instance, the high-frequency components.
The simplest filter arrangement is a time constant circuit composed of resistor and capacitor, which is used when the measurement data is an analogue signal. However, since such time constant circuit normally causes phase distortion, the filtered measurement data becomes round where the surface profile of the workpiece suddenly changes, so that the surface roughness information cannot be accurately reflected.
Another solution has been proposed, in which measurement data is converted from an analogue signal to a digital signal and the digital signal is filtered by a filtering program on a computer.
With the use of the filtering program, filtering process approximately equal to a time constant circuit can be conducted and a filtering process causing no phase distortion such as Gaussian filter can be easily conducted.
However, according to the Gaussian filter and 2CR filter, the measurement data may be deformed in the start or the end area of the measurement data and trackability for long-cycled waviness component contained in the measurement data may not be sufficient.
Further, when suddenly changing outlier is locally contained in the measurement data, the filtered data can be locally deformed under the influence of the outlier.
A spline filter is used in order to avoid the above disadvantages.
The spline filter can restrain the deformation in the start or end area of the measurement data and has excellent trackability for the long-cycled waviness component contained in the measurement data.
However, the filtering result by the spline filter can be greatly varied for a slight fluctuation of the measurement data. In order to solve the problem, in an invention disclosed in Japanese Patent Laid-Open Publication No. Hei 9-179992, a spline curve is calculated after over-sampling and filtering and a plurality of spline curves are calculated considering the magnitude of the error between the spline curve and interpolation data, the spline curves being summed up to obtain a final curve. However, since originally unexisted data is generated by the over-sampling, the method lacks reliability when being applied for measurement data such as surface texture of a workpiece.
Another invention is disclosed in Japanese Patent Laid-Open Publication No. Hei 8-278343, where high-frequency components of signal wave is removed when the power thereof exceeds a predetermined value and the signal wave is divided into a plurality of sections each of which is spline-smoothed. However, since the respective data is combined after smoothing the spline for each section, smoothness at respective nodes is hard to predict and also lacks reliability sufficient for being applied on the measurement data of workpiece surface texture.